Table of Contents
Choosing a thermoplastic starts with what the part must do. A lightweight cover, a living hinge, a sliding component, and a soft seal place very different demands on the material, even when they are all produced by injection molding.
PP may suit a low-weight cover or living hinge, while ABS is often preferred for an appearance-focused electronic housing. POM is commonly considered for low-friction moving parts, and TPE or TPU may be used for sealing or soft-touch areas. These examples create a shortlist, but the final choice still depends on the exact grade, part geometry, service environment, assembly method, mold design, and production volume.
What Are Thermoplastics?
Thermoplastics soften or melt when heated and solidify again as they cool. That behavior allows them to be processed by injection molding, extrusion, blow molding, and thermoforming. Thermosets behave differently because curing forms a permanent crosslinked structure that cannot simply be remelted.
A thermoplastic polymer can be heated again, but not endlessly without change. Excess heat, shear, moisture, or contamination can reduce strength, alter color, and destabilize processing.
How Are Thermoplastic Materials Grouped?
Two classifications are useful. Polymer structure helps explain softening, shrinkage, and cooling, while performance categories narrow the application range.
One explains behavior; the other suggests where the material may fit. Neither selects the production grade.
Amorphous and Semi-Crystalline Thermoplastics
Amorphous plastics have a mostly random molecular arrangement. Semi-crystalline plastics contain both ordered and amorphous regions, so they are not 100% crystalline.
| Factor | Amorphous thermoplastics | Semi-crystalline thermoplastics |
| Structure | Mostly random chain arrangement | Ordered and amorphous regions |
| Heating behavior | Softens over a wider range | More defined melting range |
| Typical appearance | May be transparent | Usually opaque |
| Molding behavior | Often lower shrinkage | Often higher directional shrinkage |
| Examples | ABS, PS, PC | PP, PE, PA, POM, PBT |
For molded parts, the difference becomes practical quickly. ABS and PC are often easier starting points when appearance or dimensional control matters. PP, PA, and POM need closer attention to shrinkage, flow direction, and warpage.
Commodity, Engineering, and High-Performance Plastics
Commodity plastics include PP, PE, and PS. Engineering plastics include ABS, PC, PA, POM, and PBT. High-performance thermoplastics, such as PEEK, PPS, and PEI, are considered for more severe heat, chemical, or electrical conditions.
These groups are screening tools, not quality rankings. A modified PP may suit a housing better than an unmodified engineering resin. Paying for high performance makes little sense when the part does not need it.
Common Thermoplastic Materials for Injection Molding
The following thermoplastic examples cover many molded products, from caps and housings to gears and connectors. Use the table to build a shortlist, not to approve a material.
| Material | Often chosen for | Typical molded parts | Watch for during design |
| PP | Low weight, chemicals, fatigue | Caps, covers, living hinges | Shrinkage and limited stiffness |
| PE | Moisture resistance, toughness | Containers, plugs, guards | Low rigidity and difficult bonding |
| ABS | Appearance and impact balance | Housings, panels | UV, heat, chemical exposure |
| PC | Impact strength, transparency | Guards, lenses, covers | Drying and chemical stress |
| PA | Strength, clips, wear | Brackets, gears, mechanical parts | Moisture-related dimensional change |
| POM | Low friction, movement | Gears, guides, latches | Bonding and process control |
| PBT | Electrical performance | Connectors, switches | Drying and hydrolysis |
| TPE/TPU | Grip, flexibility, sealing | Seals, grips, soft areas | Adhesion and compression set |
These thermoplastic materials examples only identify candidates. Grades with the same family name can differ in melt flow, fillers, impact modification, flame rating, and recycled content. The table narrows the field; it does not approve the material.
Why Do Thermoplastics Work Well in Injection Molding?
In thermoplastic injection molding, pellets are melted, pushed into a cavity, cooled, and ejected. One cycle can form ribs, bosses, snaps, openings, threads, and textures, which suits both cosmetic and functional parts.
The material range also supports colorants, glass fiber, flame retardants, and UV packages. Still, every resin flows, shrinks, dries, and cools differently. Switching from ABS to PP can affect gate design, cavity size, cooling, and final dimensions.
Choosing a Thermoplastic Resin for a Real Part
Begin with where the part will be used, what will touch it, and how it will be loaded and assembled. An indoor cover and a clip beside a hot engine component need different selection paths.
Data sheets help with screening, but they cannot reproduce the real wall thickness, weld lines, snap geometry, screw-boss stress, or chemical exposure. This is where a promising thermoplastic resin can fail after tooling.
What Must the Part Survive?
Will the part be dropped, flexed repeatedly, or held under load for years? Continuous heat, a transportation peak, and warmth from a nearby component are different conditions.
“Chemical resistant” is too vague for approval. An alcohol wipe is not the same as long-term contact with oil, detergent, rain, or UV. Creep and stress cracking may matter more than headline tensile strength.
How Will the Part Be Molded and Assembled?
Thin walls, long flow paths, thickness changes, and weld lines affect filling and strength. Snap-fits, screw bosses, inserts, and tight hole positions add more risk. A resin can look suitable on paper and still perform poorly in the real geometry.
Assembly and finishing matter too. A lubricated POM grade may move smoothly but resist bonding or painting. Glass-filled resin can raise stiffness while leaving visible fibers and directional shrinkage. Check overmolding, welding, printing, and adhesives before finalizing the mold.
Which Grade Should Be Approved?
PP, PC, and PA are material families, not full specifications. Grades within one family can vary in melt flow, impact modification, filler content, UV package, flame rating, color, and recycled content. A common mistake is approving the family name while leaving the production grade open.
Put the selected grade on the drawing or material specification. Alternatives need review. A “similar” grade may change snap force, crack a screw boss, shift a dimension, roughen the surface, or invalidate a flame rating tied to grade, color, and thickness.
How Material Choice Changes the Part and Mold
Changing the resin can change the part even when the CAD file stays the same. Flow affects gates and weld lines; shrinkage influences cavity size. Cooling, finish, and mold wear may shift too.
Material changes after tooling are rarely simple machine adjustments. Approve the resin before final cavity sizing, especially for tight tolerances, snaps, sealing faces, and multi-part housings.
Shrinkage and Warpage
Semi-crystalline materials generally shrink more than amorphous materials, but a data-sheet value is only a starting point. Wall thickness, gate position, packing, cooling, and flow direction all affect the result. Glass fiber can reduce overall shrinkage while increasing the difference between flow and cross-flow directions.
The consequences are familiar: a cover bows, a hole moves, snaps misalign, or a sealing face loses contact. The mold cavity is normally sized around the approved production resin. A late switch may require steel correction.
Drying and Moisture
PC, PA, PBT, and many TPU grades require controlled drying. Residual moisture can cause splay, bubbles, degradation, and reduced strength. Dew point, handling, and post-drying exposure matter too.
PA can also absorb moisture after molding, changing dimensions and stiffness. A tight fit should not be judged only in the freshly molded condition; inspection conditioning and the expected service environment need consideration.
Fillers and Additives
Glass fiber raises stiffness and strength, but it can also increase directional behavior, expose fibers at the surface, weaken visible weld-line areas, and accelerate mold wear. These are not minor formulation details.
Flame retardants, minerals, lubricants, UV stabilizers, pigments, and recycled content can change flow, color, finish, and stability. Fresh trials may still be needed.
What Does the Resin Really Cost?
Price per kilogram is only one part of the calculation. Density, drying, cycle time, rejects, mold wear, finishing, assembly, and field failures shape the final cost.
A higher-priced grade may reduce cost by supporting a thinner wall, removing coating, lowering rejects, or lasting longer. Compare the cost of a qualified part, not one kilogram of resin.
FAQs
What Are Three Types of Thermoplastics?
By application level, thermoplastics are grouped as commodity, engineering, and high-performance plastics. By structure, the common groups are amorphous and semi-crystalline. “Three types” is not the only valid classification.
What Are Five Common Thermoplastics?
PP, PE, ABS, PC, and PA are five common examples. They cover packaging, housings, transparent guards, clips, brackets, and many mechanical parts.
Are There Only Seven Types of Plastic?
No. The numbers 1 through 7 are resin identification codes, not a complete list of plastic families. Code 7 is a catch-all category that contains several unrelated materials.
Can One Thermoplastic Grade Replace Another?
Not automatically, even when both grades are labeled PP, PC, or PA. Melt flow, shrinkage, fillers, and flame performance may differ. Trial-mold, assemble, and test the replacement against the drawing.
Which Thermoplastic Is Best for Injection Molding?
Use the part requirements to decide. PP suits lightweight chemical-resistant covers; ABS suits appearance-focused housings; PA fits clips and structural parts; POM works in sliding mechanisms. Requirements decide, not popularity.
Conclusion
The right thermoplastic is not always the strongest or most expensive one. It needs to match the part’s use, structure, molding requirements, assembly method, appearance, and overall cost.
If you have a plastic injection molding project, contact HingTung to discuss your part design, material options, tooling, and production requirements.